FIG. 18 illustrates a plastic bag of prior art. The plastic bag includes a pair of side gussets 1 which are incorporated into two superposed layers of panels 2 having opposite side edges 3. The side gussets 1 extend along the opposite side edges 3 of panels 2. In addition, each of the side gussets 1 is folded into halves and interposed between the layers of panels 2 to have a folded inner edge 4 and open outer edges 5, as shown in FIG. 19 and FIG. 20. The panels 2 and the side gussets 1 are heat sealed with each other along the opposite side edges 3 of panels 2 so that heat sealed portions 6 can be formed along the opposite side edges 3 of panels 2. The plastic bag can therefore be enlarged by the side gussets 1 to obtain an increased capacity.
Furthermore, it has been recently attempted to make a fastener 7 incorporated into the plastic bag including the side gussets 1. The fastener 7 may be called a zipper and has been used generally in itself. It includes a male member formed integrally with a tape and fitted into a female member which is also formed integrally with a tape. The male and female members extend along ones of the opposite end edges 8 of panels 2 to be interposed between the layers of panels 2. The layers of panels 2 are heat sealed with the tapes of male and female members. In addition, the layers of panels 2 are heat sealed with each other along the opposite end edges 8 and 9 thereof so that heat sealed portions 6 can be formed along the opposite end edges 8 and 9 of panels 2. The plastic bag can therefore be opened and closed by the fastener 7 after cutting the panels 2 along a cutting line 10 extending between the end edges 8 and the fastener 7.
In this case, the plastic bag is problematic in that each of the side gussets 1 includes opposite end portions one of which must be intervened against the fastener 7. It is therefore required to make the panels 2 heat sealed with the end portion of each of the side gussets 1 near the fastener 7 in such a way that the panels 2 and the side gusset 1 can be closed to each other by the heat sealed portion. However, it is difficult to accomplish such the step with a sealing strength enough to be free of leakage.
It is therefore an object of the invention to provide a plastic bag including a pair of side gussets, which is arranged to be free of leakage from one end portion of each of the side gussets.